The present invention pertains a power device with protection against undesirable self-activation.
As is known, power devices are able to manage high voltages and currents. However, external causes, such as disturbances that are created on the supply networks, may lead to these quantities exceeding the maximum values that can be withstood by these devices or may even modify the electrical stability of the circuits in which these devices are inserted.
In automotive applications, power devices are required that are able to prevent malfunctioning of any nature. For example, in ABS devices an accidental disconnection may occur of the gate terminal of the power device from the control unit located on the motor vehicle. In fact, if the gate terminal of the power device remains floating, its potential may reach, by capacitive coupling, the potential applied to the drain terminal of the device itself. This causes self-activation of the power device at drain voltages that are well below the desired value, with consequent flow of current. In order to prevent this from happening and in order for the power device to be able to withstand high drain voltages, it is therefore necessary for the gate terminal never to remain floating, but to be always connected, for example, to the source terminal of the device itself.
In addition, it is necessary to keep the costs for manufacture of the power device low, by keeping the process of its integration simple to implement both in terms of masks to be used and in terms of process steps to be performed.
A known power device 1 is shown in FIG. 1 and comprises a vertical transistor of the MOS type having a gate terminal 2 and a source terminal 3 connected together via a diffused resistor 5 having a resistance R. The gate terminal 2 is also connected to a control unit 10, which is shown only schematically in FIG. 1 and comprises known driving circuits that supply a driving signal to the power device 1. The diffused resistor 5 and the power device 1 are made in one and the same chip 20. In particular, the diffused resistor 5 is formed in the proximity of the gate pad of the power device 1 by doping, with N-type impurities, a polysilicon portion previously enriched with P-type impurities.
Although this known solution is simple to implement and is economically competitive, it does not prevent undesirable turning-on of the power device 1. In fact, for proper operation of the power device 1, the resistance R must have a high value. Specifically, when the gate terminal 2 receives the driving signal from the control unit 10, the value of the resistance R should be such as not to absorb the charge supplied to the gate terminal 2. On the other hand, when the control unit 10 does not supply the driving signal, thus leaving the gate terminal 2 floating, the resistance R should have a low value so as to short-circuit the gate terminal 2 and source terminal 3 of the power device 1, thus keeping it turned off. Since the two requirements referred to above are mutually incompatible, it is not possible to have a power device that operates correctly and is protected from self-activation.
At present, in order to prevent the phenomenon of electrostatic charges, a first Zener diode 6 and a second Zener diode 7 are mutually connected in anti-series between a drain terminal 4 of the power device 1 and the gate terminal 2; in addition, a third Zener diode 8 and a fourth Zener diode 9 are mutually connected in anti-series between the gate terminal 2 and the source terminal 3 of the device itself. However, also this circuit arrangement fails to prevent self-activation of the power device 1, given that the breakdown voltage of the Zener diodes 6 and 7 must be higher than the driving voltage required for turning on the power device I during normal operation.
The disclosed embodiments of the present invention are directed at solving the technical problems referred to above by providing a power device that is protected against undesirable self- activation.
The foregoing problems are solved by a power device that includes a first conduction terminal and a second conduction terminal, and control terminal; a variable-resistance protection circuit having a first conduction terminal and a second conduction terminal connected, respectively, to the control terminal and to the second conduction terminal of the power device, the protection circuit having a resistance variable between a first value and a second value according to an operating condition of the power device.
In accordance with another embodiment of the invention, a circuit is provided that includes a power transistor having a drain terminal, a source terminal, and a control terminal to receive a driving signal; and a protection circuit having an electric static charge prevention circuit and a switch resistance circuit coupled to the drain, source, and control terminals of the power transistor, the switch resistor circuit configured to electrically connect the control terminal to the source terminal of the power transistor in the absence of a driving signal on the control terminal and in the presence of a voltage on the control terminal having a greater value than the threshold voltage of the power transistor.
In accordance with another aspect of the present invention, a method for protecting a power transistor from unwanted self-activation is provided, the power transistor having a control terminal, drain terminal, and source terminal, the method including: sensing the absence of a driving voltage on the control terminal of the power transistor; and coupling a resistance between the control terminal and source terminal of the power transistor when the voltage on the gate terminal is greater than the threshold voltage of the power transistor. Ideally, sensing the absence of a driving voltage includes sensing the presence of a voltage on the gate terminal that is greater than the threshold voltage of the power transistor.
In accordance with another aspect of the foregoing method, the coupling includes automatically switching on a switch coupled in series with a resistance between the control terminal and the source terminal of the power transistor. The switching includes having the control terminal of the switch coupled to the drain terminal of the power transistor and automatically turning on the switch when the voltage on the drain terminal increases to the threshold voltage of the switch.